Combined cycles are currently the preferred process for generating electric power on a large scale. A combined cycle includes two power generation systems: a gas turbine fired by a fuel gas, commonly natural gas that drives an electric power generator and a steam turbine powered by steam that drives a second power generator. The steam is raised with heat recovered from gas turbine exhaust gas. The gas turbine also drives a compressor that pumps ambient air into the gas turbine to burn the fuel gas stream.
Sensible heat in the exhaust gas stream from the gas turbine is recovered in a heat recovery unit by indirect heat transfer to a stream of water to convert the water to steam. The overall efficiency of a combined cycle, LHV of fuel to electric power for the gas and steam turbines is typically about 55% for air cooled turbines. Fuel heat rates for modern conventional combined cycles are typically about 6100 Btu/KWH (LHV). Typically, the gas turbine generates about 65% of the total electric power produced by the cycle and the steam turbine generates about 35%.
The exhaust steam from the steam turbine is condensed and recycled to the heat recovery unit to be converted to steam. Turbine exhaust gas, after it gives up sensible heat in the heat recovery unit is released to the atmosphere.
Ambient air is typically used with gas turbines, at about one atmosphere absolute pressure (14.7 PSIA). Firing temperatures in gas turbines currently range from about 2200 F to 2300 F. Power output increases with firing temperature but firing temperature is limited by turbine materials and need to limit NOX formation.
Improvements to the combined cycle are continually sought to improve fuel efficiency, to reduce capital cost, and to reduce pollutants released to the atmosphere in the exhaust gas, primarily NOX and CO2. The Wylie combined cycle, which is the subject of this invention, accomplishes these three objectives as well as improving efficiency and operating flexibility.